1. Field of the Invention
This invention relates to a method for continuously polarizing a thermoplastic film, and, more particularly, to a method wherein a polar thermoplastic film is passed through a polarizing zone while subjected to a high voltage, and is thus polarized therein at a polarizing temperature thereby to produce a piezoelectric thermoplastic film at a high efficiency and a high yield rate.
2. Description of the Prior Art
There is known the art of producing a piezoelectric thermoplastic film by polarizing a film made of a polar thermoplastic substance, such as polyvinylidene fluoride, polyvinyl fluoride, or polycarbonate, by the application of a high D.C. electric field at a polarizing temperature.
For instance, electrically conductive layers are deposited on both surfaces of a film made of a polar thermoplastic substance as described above, by sputtering, plating, printing, or the like suitable procedure, and a D.C. voltage is applied across the electrodes formed by the electrically conductive layers deposited on the surfaces of the film thereby to polarize the same film at a polarizing temperature. In this case, application of as high a voltage as possible can produce a film showing higher piezoelectricity so long as no insulation breakdown occurs at the polarizing temperature which is selected as a temperature higher than 50.degree. C and lower than the softening point of the film, this temperature preferably being higher than 80.degree. and lower than 180.degree. C, although a peak value thereof may be observed for rendering a highest piezoelectricity.
Since it is troublesome to polarize films one by one in a batch manner, in an industrial method, a long film deposited with electrodes on both surfaces thereof is preferably continuously passed through a polarizing zone and polarized therein by the application of a D.C. voltage and a polarizing temperature.
However, in the realization of the above described polarization, the following difficulties must be overcome.
The thermoplastic piezoelectric film has a high dielectric constant because of its polar nature, and therefore can form a capacitor which stores a high electric energy upon the application of a D.C. high voltage at the time of the polarization, the amount of this energy being proportional to the capacitance of the capacitor and the square of the applied voltage. Furthermore, the capacitance of the capacitor is proportional to the area of the electrodes made of the electrically conductive layers and also to the dielectric constant of the film substance. For this reason, a greater energy is stored on the film at the time of the polarization when the area of the film subjected to the high voltage is wider. In the case where the film constituting a capacitor during the polarization is dielectrically broken down in those portions containing electrical defects, the energy stored in the capacitor is instantaneously discharged. Due to the discharge, not only is the broken-down portion enlarged, but also the surrounding electrodes are evaporated by the heat of the discharge, whereby the effect of the defective portion is multiplied, and the production yield is remarkably lowered. When the length of the film increases with the width of the long film being constant, the damage to the film is enlarged for this reason.
Furthermore, when the polarization of the thermoplastic film is carried out at a high temperature near the softening point of the film, a slight increase in the polarizing temperature causes softening of the film, and furthermore, since even if the softening of the film does not occur, the dielectric strength of the film is lowered in accordance with the temperature rise, and the above-described dielectrical breakdown is accelerated. Such a problem is liable to occur when a thin film is polarized at a high voltage in the neighborhood of the breakdown voltage.
In addition, the large capacitance of the capacitor formed on the film causes a heavy initial current when the capacitor is connected with an electric power source, such a fact necessitating a greater capacity of the power source, and also a greater size of the device for preventing a high-voltage problem.
The film blank to be polarized is wound on a supply roll, and after polarization, the film is again wound onto a delivering roll, except in the case where the polarized film is without intermission cut into pieces of a suitable size. On these rolls, if the electrodes on both surfaces of the film blank or the polarized film are respectively continuous, electric discharge tends to occur between the electrodes on the front and rear film surfaces which come into contact with each other when the film is wound on these rolls. Although this electric discharge can be avoided by (1) interleaving an insulating film between the coils in the rolls, or (2) by polarizing two films with the use of two electrode combinations, and when rolling, placing the electrodes of a first polarity in contact with each other, thereby leaving both exposed surfaces of the combined layer with the second polarity (in this case, the outer surfaces of the two layers of the films in an overlapping relation are of the same polarity, and no electric discharge occurs when the two layers are wound on the rolls), it is not desirable from the viewpoint of hazard prevention to apply a high voltage to the parts of the film wound on the supply and delivering rolls. If the polarized film is to be cut into pieces of a suitable size, it is dangerous to do so because of the presence of a high voltage.